Operation of an internal combustion engine

ABSTRACT

A method is described for operating an engine having an afterburner and a catalytic converter downstream of the afterburner. Sufficient excess fuel is introduced into the engine to produce in the exhaust gases fuel constituents including unburnt hydrocarbons, carbon monoxide and hydrogen. Unburnt hydrocarbons are intercepted upstream of the afterburner so as to allow substantially only carbon monoxide and hydrogen to reach the afterburner. Sufficient additional air is introduced into the exhaust system to bring the composition of only the carbon monoxide, hydrogen and air to stoichiometry or leaner than stoichiometry, the resulting concentration of hydrogen and oxygen in the mixture reaching the afterburner being sufficient for ignition immediately after a cold start. 
     By removing the hydrocarbons temporarily before the exhaust gases reach the afterburner, reliable cold ignition can be achieved with lower levels of fuel enrichment.

FIELD OF THE INVENTION

The present invention relates to the operation of an internal combustionengine fitted with a catalytic converter and having an exhaust gasignition system for reducing the light-off time of the catalyticconverter.

BACKGROUND OF THE INVENTION

PCT Patent Application WO 92/22734, of which the present invention is animprovement, discloses a method of reducing the total emissions duringcold starts from an engine burning a hydrocarbon fuel and having anafterburner arranged upstream of a catalytic converter. The methodcomprises the steps of adding an excess of fuel to the enginecombustible charge and adding air to reach the engine exhaust gases toassure the presence in the exhaust/air mixture immediately after theengine has first fired of sufficient concentrations of hydrogen andoxygen to permit the resulting exhaust/air mixture to be ignitable andto burn with a steady flame in the afterburner while the latter is at atemperature close to the ambient temperature, and igniting theexhaust/air mixture in the afterburner immediately after the engine hasfirst fired.

A difficulty encountered in the above method of operating an engine isthat the mixture required during cold starts is so rich that it canresult in undesirable side effects such as plug fouling and runninginstability. For these reasons the exhaust gas ignition regime can onlybe maintained for a relatively short time after the engine has firstfired and this result in only a relatively thin slice of the catalysismatrix being heated to its light-off temperature. Thereafter it isnecessary to run in a modified regime to ensure that this slice remainshot and that the remainder of the catalysis matrix is brought to itslight-off temperature.

OBJECT OF THE INVENTION

The present invention therefore seeks to provide a method of operatingan engine that permits exhaust gas ignition to occur during cold startswith more moderate levels of fuel enrichment to the engine.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method ofoperating an engine having an afterburner and a catalytic converterdownstream of the afterburner comprising the steps of introducingsufficient excess fuel to the engine to produce in the exhaust gasesfuel constituents including unburnt hydrocarbons, carbon monoxide andhydrogen, intercepting the unburnt hydrocarbons upstream of theafterburner so as to allow substantially only carbon monoxide andhydrogen to reach the afterburner, introducing sufficient additional airinto the exhaust system to bring the composition of only the carbonmonoxide, hydrogen and air to stoichiometry or leaner thanstoichiometry, the resulting concentration of hydrogen and oxygen in themixture reaching the afterburner being sufficient for ignitionimmediately after a cold start, igniting the mixture in the afterburnerwith an ignition source, releasing the intercepted unburnt hydrocarbonsto the afterburner after the ignition and introducing sufficientadditional air into the exhaust system to bring the composition of thereleased unburnt hydrocarbons, carbon monoxide, hydrogen and air tostoichiometry or leaner than stoichiometry to ensure complete combustionwithin the exhaust system.

In the prior art proposal referred to above, oxygen was added to thesystem to bring the entire mixture reaching the afterburner tostoichiometry and for this purpose additional oxygen had to beintroduced to react not only with the hydrogen and carbon monoxide thatare vital to achieve cold ignition but also with the unburnt hydrocarboncontent of the exhaust gases. The additional amount of air introducedfor the purpose of reacting with the hydrocarbons reduced the hydrogenand carbon monoxide concentrations. For these to reach the criticallimit required for cold ignition, the quantity of fuel injected to theengine had to be increased. It should be noted in this context that theconcentrations of hydrogen and oxygen are critical in that cold ignitioncannot occur if the hydrogen concentration is lower than 3% and theoxygen concentration is lower than 6%, by volume. For reliable ignition,a hydrogen concentration of between 5% and 6% at the afterburner is tobe preferred.

In the present invention, the hydrocarbon content of the exhaust gasesis not burnt during the exhaust gas ignition (hereinafter termed EGI)phase and is instead intercepted and temporarily stored so that only thecarbon monoxide and hydrogen reach the afterburner. The amount ofadditional air necessary to achieve stoichiometry with the hydrocarbonsremoved from the exhaust mixture is therefore reduced. The reduceddilution enables a weaker mixture to be supplied to the engine cylindersduring the EGI phase while still reaching the hydrogen concentrationsnecessary for EGI. It will be appreciated that it is not so much theoxygen required to react with the hydrocarbons that creates the dilutionproblem as the remaining nitrogen contained in the additional air, whichhas four times the volume of the oxygen.

Nitrogen constitutes the most significant diluent of the combustiblegases but the exhaust gases also contain steam and carbon dioxide, thepresence of which tends to reduce the hydrogen and oxygenconcentrations. The same technique of temporary storage could be usedfor the steam and carbon dioxide, by suitable selection of a chemicaltrap, but the advantages to be gained are only of secondary importance.

The interception and storage of the hydrocarbons is preferably performedby means of a chemical trap. Chemical traps are already known and havebeen proposed for the purpose of sporing hydrocarbons until thecatalytic converter reaches its light-off temperature. These traps,unlike the trap in the present invention, are usually placed downstreamof the catalytic converter so as not to slow down the lighting-off ofthe catalytic converter and in order that the hydrocarbons stored inthem should not be desorbed prematurely before the catalytic converterhas reached its light-off temperature. After lighting off of thecatalytic converter, the traps are purged by an air flow fed back intothe exhaust system upstream of the catalytic converter. It is also onlyrecently that chemical traps have become available that are capable ofwithstanding high temperatures and capable of being positioned, as inthe present invention, upstream of the afterburner.

The interception may alternatively be purely a physical one relying onthe condensation of the hydrocarbons onto a cold surface of largesurface area. In either form of trap the hydrocarbons will automaticallybe released as the exhaust temperature rises in one case by desorptionand in the other case by evaporation.

The construction of the hydrocarbon trap may in practice be generallysimilar to that of the matrix of a catalytic. converter the differenceresiding essentially in the coating applied to the ceramic matrix. It istherefore possible to integrate the hydrocarbon trap into a catalyticconverter matrix either by the entire matrix serving the dual functionof chemical trap and catalytic converter, or by part of the matrixacting as a chemical trap while the other acts as a catalytic converter.

While the use of a chemical trap permanently in series with theafterburner is preferred, it is alternatively possible to provide ahydrocarbon trap in a bypass line the flow through which is controlledby valves in the exhaust system.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described further, by way of example, withreference to the accompanying drawing which is a schematicrepresentation of an engine having an HC trap, an afterburner and acatalytic converter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawing, an engine 12 has a intake manifold that includes anintake air flow meter 22, a butterfly valve 24 and fuel injectors 20.The engine also has an exhaust system comprising a down pipe 14 leadingto a combined HC trap 10, afterburner 16 and catalytic converter 11. Thecombined converter is formed of two bricks 10, 11 separated from oneanother by the afterburner chamber 16 in which there is arranged anigniter in the form of a spark plug 18.

The engine also has an exhaust gas oxygen sensor 38 that is used duringnormal running conditions in controlling the fuelling of the engine, andan air pump 30 acting as a source of additional air for injecting airinto the exhaust system through a valve 32, without the air passingthrough the engine combustion chambers.

The system as so far described is generally similar to that described inPCT Application WO92/22734, differing from it only in the constructionof the combined HC trap, afterburner and catalytic converter. For thisreason, the reader is referred to the earlier application for a fullerdescription of the system and only a summarised explanation will begiven within the context of the present invention.

Under normal conditions, that is to say when the engine is hot, the airpump 30 is not operated and the exhaust gas oxygen sensor 38 is used toensure that a stoichiometric fuel to air ratio is supplied to thecombustion chambers. The brick 11 of the catalytic converter acts as athree way catalyst and at its normal running temperature it ensures thatthe carbon monoxide, oxides of nitrogen and hydrocarbons still presentin the exhaust gases react with one another to produce only carbondioxide, nitrogen and steam.

During start up from cold, the catalytic converter does not operate andan EGI regime is adopted to bring the brick 11 up to its light-offtemperature as quickly as possible. During EGI, the injectors 20 injectmuch more fuel than is required for stoichiometry and this over fuellingcreates hydrogen and carbon monoxide in the exhaust gases. The air pump30 is operated and the flow of air is regulated by the valve 32 so thatstoichiometry is now achieved within the exhaust system and, on reachingthe afterburner chamber 16, this mixture is ignited by means of thespark plug 18 to heat the brick 11. At the same time, the combustionwithin the afterburner 16 itself consumes the noxious emissions thatwould otherwise have escaped past the cold catalytic converter brick 11.

In the prior proposal, the brick 10 in the catalytic converter was alsoconstructed as a three way catalyst. In the present invention, however,the brick 10 is constructed to include a hydrocarbon trap that actsduring initial start up to adsorb the hydrocarbons in the exhaust gasesand to prevent them from reaching the afterburner chamber 16. Theconstruction of a hydrocarbon trap can physically resemble theconstruction of a catalyst matrix, the difference residing in thewashcoat applied to the ceramic matrix. It is possible therefore to formthe first of the two bricks exclusively as a hydrocarbon trap or as acombined hydrocarbon trap and three way catalyst.

The effect of positioning a hydrocarbon trap upstream of the afterburneris that less oxygen is now required to be introduced into the exhaustsystem to achieve a stoichiometric mixture within the afterburner 16.Because less diluent enters the exhaust system, less hydrogen needs tobe produced by the combustion in the engine to reach the concentrationsessential for cold ignition. In this way, the invention allows exhaustgas ignition to occur with lesser levels of fuel enrichment.

By reducing the over fuelling to more tolerable levels the inventionenables the EGI regime to be maintained for a more prolonged period.Furthermore the heat generated within the afterburner is less intensebecause only hydrogen and carbon monoxide are burnt. Therefore, thesecond brick 11 is heated by a gentler flame lasting a longer time whichprovides deeper penetration of the slice reaching its light-offtemperature into the matrix and also reducing the risk of the front faceof the catalytic matrix being damaged by overheating.

Adsorption of the hydrocarbons tends to take place when the first matrixis cold and removal of the hydrocarbon content of the exhaust gases isalso assisted by condensation of the hydrocarbons on the large coldsurface area of the capillaries within the first brick 10. As thetemperature of the matrix 10 rises, the adsorbed fuel is desorbed andthe condensed fuel is evaporated. At this time more oxygen is introducedinto the exhaust system to restore the mixture within the afterburnerchamber 16 to stoichiometry. By this time, the flame within theafterburner will have been extinguished by reducing the fuel enrichmentto the engine and instead the gases will react exothermically with oneanother in the second brick 11 to spread the slice that has reachedlight-off temperature until it should occupy the entire matrix 11.

There are currently available hydrocarbon traps that will survive in theposition of the brick 10 in the catalytic converter housing but it isalternatively possible to use a lower temperature trap and to positionit within a bypass line through which the exhaust gases are diverted byvalves during the EGI regime.

As well as acting as a hydrocarbon trap and catalytic converter, thefirst brick 10 in the configuration illustrated assists in ensuring alaminar flow of gases for improving the combustion in the afterburnerand also acts as a flame trap to prevent the flame within theafterburner from spreading back up the exhaust pipe.

It can be seen from the foregoing that the present invention improvesover the earlier EGI proposal by reducing the levels of fuel enrichmentnecessary to achieve a flame in the afterburner without adding to thecomplexity and the size of the system.

I claim:
 1. A method of operating an engine having an afterburner and acatalytic converter downstream of the afterburner comprising the stepsof introducing sufficient excess fuel to the engine to produce in theexhaust gases fuel constituents including unburnt hydrocarbons, carbonmonoxide and hydrogen, intercepting the unburnt hydrocarbons upstream ofthe afterburner so as to allow substantially only carbon monoxide andhydrogen to reach the afterburner, introducing sufficient additional airinto the exhaust system to bring the composition of only the carbonmonoxide, hydrogen and air to stoichiometry or leaner thanstoichiometry, the resulting concentration of hydrogen and oxygen in themixture reaching the afterburner being sufficient for ignitionimmediately after a cold start, igniting the mixture in the afterburnerwith an ignition source, releasing the intercepted unburnt hydrocarbonsto the afterburner after the ignition and introducing sufficientadditional air into the exhaust system to bring the composition of thereleased unburnt hydrocarbons, carbon monoxide, hydrogen and air tostoichiometry or leaner than stoichiometry to ensure complete combustionwithin the exhaust system.
 2. A method as claimed in claim 1, whichfurther comprises intercepting steam and carbon dioxide to reducefurther the proportion of non-combustible diluents in the gases reachingthe afterburner.
 3. A method as claimed in claim 1, which comprisesplacing a chemical trap permanently in series with the afterburner andforming the trap of a material capable of withstanding the normalexhaust gas operating temperatures.
 4. A method as claimed in claim 1,which comprises placing a chemical trap in a by-pass line through whichexhaust gases are diverted only during cold engine operation.
 5. Amethod as claimed in claim 3, which comprises forming the chemical trapintegrally with the first brick of a two-brick catalytic converter andforming the afterburner in the chamber between the two bricks.